iTunes rewrites two files, iTunes Library and iTunes Music Library.xml, every time a song is played or other changes (like adding a rating) are made to the iTunes library. When you have a large number of tracks, these two files can total anywhere from 50MB to 100 MB. So if you play just 10 tracks, that's up to a gigabyte's worth of data being rewritten.

FileVault won't reclaim this space until logout, so this is potentially a quick way to eat up free space on a volume. One solution is to move the iTunes folder to a different (unencrypted) location, and have an alias from the original Music folder. On my machine, for instance, in my user's Music folder, I have an iTunes alias pointing to /Users -> Shared -> Music -> iTunes. I've been running this for about 6 months with no ill effects.